This invention relates generally to zero crossing detector circuits and more particularly to such a circuit usable in conjunction with an electronic ignition system for controlling the spark timing thereof.
In an electronic type ignition system it is desirable to produce a spark at the spark plugs of the system at substantially fixed times which vary only minimally. In these systems the magnetic generator in the distributor normally provides a voltage varying from some positive amplitude to some negative amplitude; different magnetic generators producing different wave shapes. The voltage provided by the magnetic generator conventionally renders a transistor or the like semi-conductor component conductive which in turn operates other circuitry to produce the spark.
The biasing voltage required to render a transistor conductive varies with temperature so that in some instances the transistor requires a greater voltage than in others to become biased into conductivity. When the latter occurs, the spark is not produced at the same time relative to the movement of the magnetic generator and as such the timing of the engine with which the ignition system is being used is changed.
While it is known in the art to temperature control a transistor by providing in the biasing circuit a semi-conductor diode fabricated of a material similar to that of the transistor which maintains the voltage at the base-emitter junction of the transistor substantially constant, the dynamic impedance of the diode is higher than the dynamic impedance of a diode connected transistor as is provided in the present invention and, as such, raises the series impedance at the base of the controlled transistor resulting in poorer noise immunity. Additionally, the thermal tracking characteristics of a diode do not match the thermal characteristics of the controlled transistor as well as a diode connected transistor, as is provided in the present invention, resulting in inferior insensitivity to voltage variations caused by temperature changes and power supply fluctuations.